I'm obsessed with thrillers and cop dramas, mostly from the UK but from other countries as well, but I realise not everyone will be familiar with all the terms used, so here is an explanation of some of them.

CID = Criminal Investigation Department. Most people with 'detective' in the title will belong to this. It's used for murders and other complex investigations.

Custody Sergeant = this person will be on duty when a suspect is taken into custody to take their details etc.

Custody suite = A secure area in a police station where people are taken after being arrested.

DC = Detective Constable. The lowest ranked type of Detective who will be mostly doing work such as knocking on doors etc. All the detectives and non detectives have equivalent but parallel 'ranks'.

DS = Detective Sergeant. The lowest 'supervisory' rank who will be managing a team of DCs. They will report to a DI.

DI = Detective Inspector - will be running murder investigations, as well as interviewing suspects and other work. They will be the ones standing in front of the board of photos of suspects etc

DCI = Detective Chief Inspector. They will be heading up the department and in charge of very complex inquiries, possibly out of several police stations. All of these will still be doing work such as interviewing suspects etc, although the DCI less so.

Family liaison officer = A police officer that is assigned to 'look after' the families of people involved during the investigation (usually the victims but could be the perpetrators or sensitive witnesses). This person is supposed to keep the family informed of developments in the investigation but will also inform the department if they have suspicions.

PC = Police Constable. The lowest rank of 'general' police officer who won't necessarily be investigating crimes but have other duties.

Old Bailey = The most well known court in the country which hears many (but not all) high profile cases.

Police Commissioner = The person who oversees the entire force in an area. In recent years they have been elected.

Police National Computer = A database on which DNA etc and info regarding criminal records, etc, is held. Controversially it can also include details of those who were arrested and cleared of a crime.

Superintendent = The boss.

The Met = The Metropolitan (London) police. They are the largest most well known force in the country, and their headquarters is Scotland Yard. They have been hit by scandals in recent years.

TVP = Thames Valley Police, which operate in Oxford, Berkshire, Buckinghamshire etc.

Other forces include Greater Manchester Police, West Yorkshire Police (Heather's force), British Transport Police (which investigate crime committed in public transport), Avon and Somerset Police, and more.

I might add to this as it goes on :D  and of course please tell me if I've got something wrong.